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The coordination between cell growth and division is a highly regulated process that is intimately linked to the cell cycle. Despite a wealth of knowledge about cell growth and division, little is known about the mechanisms that regulate the cell cycle in response to cell size.

Efforts to identify a signalling system that measures cell size and conveys that information to the cell-cycle machinery have been unsuccessful. Instead, we propose that size control is an intrinsic function of the basic cell cycle machinery. Using both population-level and single cell assays we show that two key positive regulators of the G2/M transition—Cdc13, the B-type cyclin that forms CDK with the Cdc2 kinase, and Cdc25, the phosphatase that activates CDK—accumulate in a size dependent manner. That is, unlike most proteins which maintain a constant concentration as cells grow, Cdc13 and Cdc25 increase in concentration as cells get bigger. Since smaller cells accumulate less of the activators and larger cells accumulate more, we propose that they provide a size-dependent mechanism to trigger cell division when cells reach a threshold concentration of these activators.